fs: Convert NFFS partition to a generic one

The NFFS partition at the end of flash is also useful for any other
file system or even the Flash Circular Buffer (FCB). Rename the
partition from 'nffs_partition' to 'storage_partition' and make it
depend on a new hidden Kconfig entry which the relevant users will
select (such as NFFS and FCB).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>